The spider crab is a commonly seen crab in Rhode Island waters.Courtesy: University of Charleston, S.C.

Spider Crab (Libinia emarginata)

Color: Body is mud colored; claws are whitish-yellow
and stand out from the rest of the crab.Size: Carapace is up to 4 inches wide. Males
grow larger than females and can be 9 inches from claw to claw when stretched
out.Habitat: Entire Bay bottom, rocky shores, harbors,
eelgrass beds, and pilings.Seasonal appearance: All year.

Description

The spider crab is one of the most widely recognized of all Rhode Island marine
inhabitants. This long-legged crustacean is in fact a crab and not a spider
as its name suggests. The carapace of this crab is round and spiny, with nine
small spines running down the center of the back. Its tapered snout and short
eyestalks are located on the rostrum, or tip of the carapace, which extends
out in a shallow V-shaped notch. Spider crabs range in size, with adult males
growing larger than juveniles and females.

The legs and pincers of the male spider crabs can be nearly twice as long
as those of the females. The spider crab's claws are different from those of
other crabs. The claws are narrow, long pincers that are slow and not as strong
as many other crabs; however, the larger males have big claws that can deliver
quite a pinch.

Life History and Behavior

Spider crabs use the ends of their claws to scoop up bits of detritus and algae.
Spider crabs are non-threatening and somewhat lethargic scavengers. They have
poor eyesight; however, they do have sensitive tasting and sensing organs on
the end of each walking leg. This allows them to identify food in the water
or in the mud as they walk over it.

Like all other crabs, spider crabs molt to grow. The females stop molting after
they become sexually mature and remain the same size for the rest of their lives.
When molting, spider crabs will cling to the tops of eelgrass close to the water's
surface.

Spider crabs attach bits of algae,
shell, and seaweed to the many fine, sticky hairs covering their bodies
for camouflage.

Special Notes

One of Rhode Island's living fossils, the spider crab's remarkable adaptability
has enabled it to thrive practically unchanged for hundreds of millions of
years.

Spider crabs are highly tolerant of pollution and can live in harbors where
there is oil and other pollutants. They can also tolerate low oxygen, or eutrophic
environments where there are generally few inhabitants.

When startled, spider crabs will wave their pincers over their heads in
a beckoning gesture to warn off potential predators.

Adapted from The Uncommon Guide to Common Life on Narragansett Bay.
Save The Bay, 1998.